An enclosed air conditioning unit includes a filter section and a cooling section through which intake air passes before being discharged into a space within a building. The orientation of the filter section and cooling section is substantially vertical, and the airflow path through the filter section and the cooling section is substantially horizontal, resulting in reduced face velocities across these components, thereby increasing filtration efficiency and cooling effectiveness, while allowing the physical size and configuration of the air conditioning unit's enclosure to be the same as or smaller than the enclosures for conventional air conditioning units having comparable or lower performance.